[YorkieMother]

You are telling us that the dog never sits ever on its butt. He never once in the whole day ever just plants his butt on the floor.

If he sits without you asking try a capture clicker method instead. Walk up and pass him a treat when he sits on hs own or tos him on after a click.
He will soon figure it out that the butt on the ground is what you want.

Then and only then do you add the hand signals and the words.
Action is always taught ahead of the word. Dogs do not know sit means anything at all until he has leanred the action

JL